Java Classes & Packages Quiz

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Questions and Answers

What method is invoked to trigger the Dead/Destroyed State in an applet?

destroy() (correct)

quit()

terminate()

finish()

Which of the following correctly describes a Frame in Java AWT?

A Frame can contain other components and has a title bar. (correct)

A Frame can only contain buttons and text fields.

A Frame cannot have menu bars.

A Frame is the same as a Panel but with a title bar.

Which statement about Java AWT components is true?

AWT components are platform-independent.

AWT is not used for GUI applications.

AWT components are lightweight and use minimal resources.

AWT components utilize the resources of the operating system. (correct)

What is the primary function of the paint method in the HelloWorldApplet class?

To display string graphics. Signup and view all the answers

Which of the following correctly describes a Panel in Java AWT?

A Panel can contain buttons and text fields without a title bar. Signup and view all the answers

What is the purpose of using a Java package?

To group similar types of classes and interfaces Signup and view all the answers

Which of the following is NOT a built-in Java package?

java.custom Signup and view all the answers

How is a string object created implicitly in Java?

By using a string literal Signup and view all the answers

What is one of the main advantages of using applets?

They provide less response time by running at the client side Signup and view all the answers

Which of the following correctly demonstrates explicit string object creation in Java?

String s = new String('Hello world'); Signup and view all the answers

What does the method msg() in the class A do?

Prints 'Hello' Signup and view all the answers

What defines the nature of applets?

They are small Java programs embedded in web pages Signup and view all the answers

Which statement accurately describes string literals in Java?

String literals automatically create string objects Signup and view all the answers

What is the main purpose of the `setVisible(true)` method in the example using inheritance?

To ensure the frame is displayed to the user. Signup and view all the answers

Which of the following event classes is NOT mentioned in the provided content?

TouchEvent Signup and view all the answers

In the example using association, which of the following is a necessary step for displaying the button?

Adding the button to the frame using <code>add()</code> method. Signup and view all the answers

What does the `setLayout(null)` method accomplish in the frame?

It allows for absolute positioning of components. Signup and view all the answers

Which listener interface is associated with handling keyboard events?

KeyListener Signup and view all the answers

What method is NOT called during the lifecycle of an applet?

run() Signup and view all the answers

What is the state of an applet after invoking the destroy() method?

Dead/Destroyed Signup and view all the answers

Which statement is true regarding the main() method in applets?

Applets do not use main() method. Signup and view all the answers

Which method is responsible for displaying the applet's output?

paint() Signup and view all the answers

What happens during the Idle state of an applet's lifecycle?

The applet stops running. Signup and view all the answers

Which of the following restrictions applies to Java applets?

They cannot read or write local files. Signup and view all the answers

When is the stop() method invoked in an applet's lifecycle?

When the applet moves from running to idle state. Signup and view all the answers

How often is the start() method called during the applet lifecycle?

More than once after initialization. Signup and view all the answers

What method is used to add an action listener to a Button in Java AWT?

addActionListener(ActionListener a) Signup and view all the answers

In the AEvent class, what happens when the button is clicked?

The text field displays the message 'Welcome'. Signup and view all the answers

Which of the following components can add text listeners according to the Java AWT framework?

TextField Signup and view all the answers

What does the actionPerformed method require as a parameter?

ActionEvent e Signup and view all the answers

What does the following line of code do: tf.setBounds(60,50,170,20);?

Sets the position and size of the text field on the frame. Signup and view all the answers

Which component does NOT have an action listener method according to the provided content?

TextArea Signup and view all the answers

In the ButtonExample class, what is the purpose of the line 'final TextField tf=new TextField();'?

To create a new TextField object that captures user input. Signup and view all the answers

What is the significance of 'setLayout(null)' in both examples?

It enables absolute positioning of components within the frame. Signup and view all the answers

Study Notes

String

String is a class, not a primitive data type in Java.
Two ways to create string objects: Implicit and Explicit.
Implicit: Java automatically creates a string object using a string literal (e.g., String s = "Hello world";).
Explicit: Use the new operator to instantiate a string object (e.g., String s = new String ("Hello world");).

Package

A Java package is a group of related classes, interfaces, and sub-packages.
Two types of packages: Built-in and User-defined.
Built-in packages include java, lang, awt, javax, swing, net, io, util, and sql.
Advantages of packages:
- Categorize classes & interfaces for easier maintenance.
- Provide access protection.
- Prevent naming collisions.

Java Applet

Applet is a small Java program embedded within web pages.
Transported over the internet from web servers to client computers.
Enhance web pages with rich media and support application delivery.
Applet advantages: Client-side execution (reduced response time), security, and platform independence.

Differences Between Applet and Application

Both are Java programs, but applets have security restrictions.
Applet:
- Does not use the main() method; uses init(), start(), paint(), stop(), and destroy().
- Embedded in web pages and executed in browsers.
- Cannot access local files or communicate with other servers.
- Cannot run programs from the local computer.
- Restricted from using libraries from other languages.

Applet Lifecycle

Applet inherits behaviors from the Applet class.
States:
- Initialization: Invokes the init() method once.
- Running: Invokes the start() method multiple times, including automatically after init() execution.
- Display: Invokes the paint() method multiple times for displaying output.
- Idle: Invokes the stop() method multiple times when the applet is not running.
- Dead/Destroyed: Invokes the destroy() method once when the browser is closed.

Java AWT (Abstract Window Toolkit)

API for creating GUI or window-based applications in Java.
The java.awt package provides classes for creating GUI elements like TextField, Label, TextArea, RadioButton, CheckBox, Choice, List, etc.
AWT components are platform-dependent, meaning their appearance depends on the underlying operating system.
AWT is heavyweight; its components rely on the resources of the OS.

Java AWT Hierarchy

Container component can contain other components like buttons, textfields, labels, etc.
Container classes include Frame, Dialog, and Panel.
Window is a container without borders and menu bars.
Panel is a container without title or menu bars.
Frame is a container with a title bar, menu bars, and other components.

Event Handling

Event - changing the state of an object (button click, mouse drag, etc.).
The java.awt.event package provides event classes and listener interfaces for handling events.
Example classes: ActionEvent, MouseEvent, KeyEvent, ItemEvent, TextEvent.
Example listener interfaces: ActionListener, MouseListener, MouseMotionListener, KeyListener, ItemListener, TextListener.

Steps to Perform Event Handling

Register/Attach Listener: Register the component with a listener using provided methods:
- Button: public void addActionListener(ActionListener a){}
- MenuItem: public void addActionListener(ActionListener a){}
- ... (similar methods for other components)

Java Event Handling Using `ActionListener`

The ActionListener interface handles actions performed on components like buttons or menu items.
Example code:
- Implement the ActionListener interface in a class.
- Create an instance of the component (Button, TextField, etc.).
- Use addActionListener(), passing the current object (implementing the listener), to attach the listener to the component.
- The actionPerformed() method of the listener will be invoked when the event occurs.

Java AWT Button Example with `ActionListener`

Code demonstrates creating a button with text "Click Here" and a text field.
When the button is clicked:
- The actionPerformed() method of the ActionListener updates the text field with "Welcome to Javatpoint."
- Highlights the process of:
  - Creating a button and text field objects.
  - Setting their positions and sizes.
  - Adding the button and text field to a frame.
  - Adding an action listener to the button.
  - Setting the frame's size, layout, and making it visible.

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Description

Test your knowledge on Java classes, packages, and applets. This quiz covers key concepts such as how to create string objects and the advantages of using packages in Java programming. Challenge yourself to reinforce your understanding of these essential Java components.